The present invention relates in general to conditioning of seed cotton for ginning, and specifically for drying of high-moisture seed cotton, and more particularly to seed cotton drying systems involving a tower dryer wherein seed cotton descends along a continuous restricted zigzag path defined by shelflike partitions in a casing forming the tower dryer, with the seed cotton being impelled along the zigzag path by a high-velocity stream of heated air.
Heretofore, the harvesting of high-moisture seed cotton has been recognized as a quality problem as early as the 1930's and much work has been carried on in developing methods of drying the seed cotton prior to ginning the cotton, including work carried on by the U.S. Department of Agriculture Ginning Laboratory at Stoneville, Mississippi. Work by the U.S.D.A. Ginning Laboratory resulted in what has become known as the parallel flow tower dryer, which for many years has been the most prevalent type of seed cotton drying employed at the pre-ginning stage in cotton ginning installation.
Those early efforts were directed at lowering the moisture content of the seed cotton enough to allow the gin to produce a smooth sample. High-moisture seed cotton resulted in rough preparation of the lint. The "preparation" of the lint was a very important quality factor, along with "Grade" and "Staple". More recently, the wide use of lint cleaners which involve a combing action, which are customarily used in gin plants in association with gin stands or between the gin stands and the battery condenser, result in smooth preparation due to the combing action of the lint cleaners under almost all moisture conditions and the "preparation" factor is no longer as significant a factor in quality as it once was.
Prior to World War II, a number of gins were equipped with dryers to improve the preparation, but in those cases, except for the arid areas of Texas and Oklahoma, very little seed cotton cleaning was used in addition to the drying. The labor shortage of the post-World War II period made the careful hand harvesting of seed cotton impractical, and mechanical harvesting of cotton developed at a very rapid rate. This usually resulted in the adding of moisture to improve the efficiency of the seed cotton harvester, resulting in a significant increase in the need for drying at the gin plant prior to the ginning of the seed cotton. Also, the mechanical harvesting method resulted in much more foreign matter being brought to the gin. It was found that drying the lint to a low-moisture content improved the efficiency of the seed cotton cleaning achieved at the gin, and gins across the country rapidly installed two stages of drying and began to use more and more heat energy to produce better grades of cotton notwithstanding the mass of leaf trash and stems in the high-moisture seed cotton resulting from mechanical harvesting methods. In fact, use of three stages of drying in gins to improve seed cotton cleaning is not uncommon today.
The parallel flow tower dryer has become the most commonly and successfully used of the drying apparatus by the ginning industry. The parallel flow tower-dryer method involves the use of direct-fired heaters usually rated at from 3,000 B.T.U. up using natural gas, liquid propane gas or oil, for heating the conveying air designed to impel the seed cotton through the tower dryer. This air for drying and impelling the seed cotton passes through the heater from a suitable blower, picks up the seed cotton to be dried from a rotary airlock under a feeder, and conveys it to the top of the parallel-flow tower dryer. The tower dryer involves a plurality of parallel vertically spaced shelves which alternately extend from one end-wall of a vertically elongated casing to a location near but spaced from the opposite endwall to define a zigzag or labyrinth path descending from the top to the bottom of the tower. The heated air conveys the cotton along the shelves of the tower, dropping it from one shelf to the next and tumbling the seed cotton as it changes directions at the end of the shelf. As the seed cotton reaches the bottom of the tower dryer, the conveying air carries it through a duct to an air-separating unit where the drying air is separated from the cotton and discharged to the atmosphere.
An important factor in the efficiency of the tower-dryer system is the velocity of the air over the shelves, referred to commonly as "shelf velocity". Obviously, the shelf velocity has to be high enough to convey the seed cotton across the shelf and, unfortunately, this required velocity varies with the density of the cotton which is a function of moisture content. The best efficiency is obtained at the lowest shelf velocity which will convey the cotton along the shelves. In the early stages of development of the tower dryer, it was found that about 40 cu. ft. of air per pound of material was optimum. At this ratio, shelf velocities as low as 900 feet per minute were very successful. This resulted in low static pressure losses across the tower and only moderate air temperatures were required.
Of course, such tower-dryer systems were expected to handle only damp cotton, varying in lint moisture from about 8 to 10%. As opposed to this condition, modern high-capacity gin plants are set up to handle maximum moisture contents of from 15 to 20%, although only a small percentage of the cotton being dried has such a high moisture content. This requires shelf velocities in the range of about 3,000 feet per minute to successfully convey the cotton over the shelves at ratios of 25 to 30 cu. ft. of air per pound of material. At these velocities, the cotton is exposed to the drying air for a very short period. To compensate for this short exposure, higher temperatures are used, resulting in excessive consumption of fuel.
An object of the present invention is the provision of a tower dryer system which will reduce energy and fuel consumption and achieve higher efficiency drying of the seed cotton, thereby achieving significant economies in the initial cost of seed cotton drying systems because of reduced horsepower requirements for air-propelling equipment and achieving economies in operation due to greater conservation of heat energy.
As a means of attaining this object, I have devised a parallel flow tower dryer which provides for a high-shelf velocity over approximately the upper one-third of the shelves, which will lower the density of the seed cotton material being handled sufficiently to permit a reduced velocity over the next approximately one-third of the shelves. The reduced velocity in this middle approximately one-third of the shelves is accomplished by a wider shelf spacing. The lower approximately one-third of the shelves has a still wider shelf spacing for a still lower velocity, since the seed cotton continues to become less dense as the air absorbs the moisture.
Further, I have attained greater heat conservation and heating efficiency in the system by providing a means for heating the shelf surfaces of the shelves within the dryer, preferably by a separate system of hot air. One innate weakness of the present tower-dryer structure is the rapid cooling of the drying air as it absorbs moisture. When high-moisture cotton is introduced into the drying and conveying air, it is rapidly cooled by evaporation, as well as by radiation losses. Due to the relatively high initial temperature of the air which enters the cotton drying and conveying system upstream of the tower dryer, the moisture transfer is very rapid in the ductwork to the tower and in the upper or first group of shelves of the tower encountered by the heated air, but unless extremely high temperatures are used to pick up the cotton the temperature drop through the system is so great that the moisture transfer in the lower regions of the dryer drops below that necessary to obtain sufficient drying. The alternative is an impractically long system or a second and third stage of drying.
By providing a hot-air chamber internally of each shelf or horizontal divider partition in the tower dryer through which hot air is circulated, the conveying air for the seed cotton passing through the dryer is heated by conduction from the heated shelf surfaces so that a generally uniform temperature can be maintained in the tower from top to bottom. The source of heated air to be supplied to the hot-air chambers provided internally of the shelves can be the same heating source as that used for heating the conveying air or a separate source of heat for the shelves can be provided. By keeping the temperature of the conveying air up to a point which will provide good moisture-transfer rate throughout the tower, the efficiency is greatly improved, and proper efficient moisture transfer can be accomplished by use of such a hot-shelf dryer construction without having excessively high temperatures at the inlet of the tower.
In tower-dryer systems presently in use, it is common practice to separate the drying air from the seed cotton at a temperature in the range of about 150.degree. to 175.degree. F. and discharging this separated air to atmosphere. Very rarely is the relative humidity of this air greater than about 10 to 15% which, of course, means that a significant percentage of the heat energy or B.T.U.'s in this air could be saved by recirculating a part of the air.
I therefore propose to improve the efficiency of the system by recirculating a portion of the heated dry air based on maintaining a fixed amount of moisture in the drying air as it is separated from the cotton, so that the drying air may be recirculated until its moisture content reaches a predetermined level.
Thus another object of the present invention is the provision of a novel tower dryer structure for drying seed cotton, wherein the shelves of the tower dryer provide internal chambers which are supplied with heat to heat the shelf surfaces in such manner as to conduct heat from the heated shelf surfaces to the conveying air or reduce heat loss from the conveying air to the shelves to reduce the rate of cooling of the air for heating and conveying the seed cotton and maintain a more uniform temperature from top to bottom of the tower.
Another object of the present invention is the provision of a novel seed cotton tower dryer of the type described in the preceding paragraph, wherein a relatively higher shelf velocity is provided for in the upper portion of the dryer, while a selectively reduced shelf velocity is provided in the mid-portion by increasing the spacing of the shelves, and a still lower shelf velocity is provided in the lower portion by a still wider shelf spacing.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.